Power Source

Explosive materials require energy input to begin the decomposition reaction. The amount of energy required to initiate this reaction determines the explosive’s sensitivity. The sensitivity of a material helps determine its usability. If a material is very sensitive to shock, heat, or friction, it may be too unstable and dangerous for use. Pure nitroglycerin is an example of a highly sensitive explosive. This sensitivity limited its usefulness until a novel production process resulted in a less shock-sensitive state and the creation of dynamite. A few chemicals are so incredibly sensitive that they are unstable in all environments. Nitrogen triiodide, for example, can be detonated by the energy imparted simply from alpha radiation, seemingly spontaneously, rendering it too dangerous for handling or practical use.

Explosives relevant to this chapter are detonated after being supplied with energy, typically in the form of heat, shock, or friction. Energy must be introduced into the system first, and, historically, this energy was supplied from a flame used to light a safety-fuse, such as the type seen in common fireworks. Later, the magneto “dynamite plunger” was invented and later made widely famous by cartoons. Its advanced system generates an electrical current instead of a flame, transmitted to a primary charge, which, in turn, sets off the main charge. This revolutionary design eliminated many of the safety issues associated with gunpowder-based safety fuses. As technology progressed, new circuitry was developed that allowed more options for bomb makers compared with a fuse or a hand-powered generator. Most IEDs now use energy stored within batteries to initiate explosions, allowing the creativity of bomb makers to blossom and develop new types of triggers.

Switch/Trigger

A bomb explosion needs to be “set off” in some way. This event is known as triggering the device. When triggered, energy enters the circuit and is transferred to the explosive. The simplest systems use a fuse, which then directly provides heat to the main charge. More complex devices with electrical power sources function on a closed circuit of electricity, using wires to connect the battery to an initiator, with a switch/trigger placed in the circuit to keep the flow interrupted. In electrical engineering terms, a switch is defined as a device for making, breaking, or changing the connections in an electrical circuit. Activating the switch/trigger, closes the circuit and allows electricity to flow uninterrupted, as happens when a light switch is turned to the on position. The energy from the electrical signal then activates an initiator, such as an electrical filament or a blasting cap, which then leads to the explosion. Many types of triggers exist, and multiple numbers and types can exist within a single device, tailored to the bomb makers desired purposes.

IEDs can also be defined by how they are detonated: by command, by time, or initiated through actions by the victim. In command detonation, someone observing the target area triggers the IED by either a wire directly connected to the device or by remote control through wireless devices such as cell phones, pagers, remote car door openers, or garage door openers. An IED can also have a timer built in as a switch, allowing a bomb to be placed well ahead of detonation, and the operator to escape to a safe distance. Finally, a victim-operated IED, also known as a “booby trap,” detonates when a person or vehicle triggers the initiator (e.g., trip wire, pressure plate, pressure-release mechanism, or light-sensitive devices such as motions sensors, etc.).

Command switches are the simplest, merely requiring the push of a button. This action can be taken remotely or proximally, as seen in a suicide bomb vest. Increasingly complicated trigger designs exist in multitude. In a “time bomb” the switch is a timer or clock that, when a predetermined time is reached, the circuit closes. Instead of the bell or alarm sounding, the electricity is transmitted to the bomb initiator. The construction of cell phone, remote control, and other remotely detonated bombs uses a similar method of electricity diversion from benign parts to switch components.

Bomb makers may use the cell phone as both the power source and trigger by incorporating the phone vibrator into the circuit. When the phone vibrator is activated via alarm, call, or text, the power flows from the phone battery, through the vibrator, to the initiator. During The Troubles in Northern Ireland during the twentieth century, a period of time when there was much terrorist activity and bomb-making and disposal technologies made rapid advancements, new “antihandling” features were built into the devices to prevent movement or disarming. Mercury “tilt” switches, similar to those seen in thermostats, contain mercury that if tilted in a particular direction will contact two electrodes and complete a circuit between them causing detonation if the IED is moved.

Initiator

The initiator is the component of the bomb that directly transfers the energy (shock, heat, and friction) from the power source directly into the main charge, leading to combustion. The type of initiator required for successful explosion depends on several factors. The primary determinant is the sensitivity of the main charge and how much energy is required of the initiator to begin the explosion. If a sensitive main charge such as gunpowder is used, a simple burning fuse may provide sufficient energy; for example, in IEDs like those used in the Boston Marathon Bombing in 2013, the initiator is the exposed filament of a simple household Christmas tree light. When initiated, electricity causes the filament to glow and give off enough heat to ignite the LE in which the filament is buried.

Some explosives require much more energy to detonate. Therefore, a simple fuse or light filament will not suffice, in which case, a more powerful initiator must be used. The prime example is the commercially available “blasting cap” used in mining and military applications. In this case, the initiator is itself a small explosive device that is embedded into the main charge. When detonated, the cap provides a large amount of energy sufficient to detonate the main charge. Explosives are designed in this way to prevent accidental detonation during transport and handling. In fact, some explosives will only detonate in response to one type of energy, such as shock instead of heat. In fact, plastic explosives such as Compound 4 will burn without exploding, as the fire does not impart sufficient heat energy to trigger detonation. This property was useful to soldiers in Vietnam who would allegedly safely burn their C-4 as fuel for warming food when regular fuel supplies became scarce.

Explosive Material

When creating a bomb, first and foremost an explosive material is required. The amount of material used is proportional to the resulting energy released. HEs generate different blast physics than LEs do. The results are different levels of destruction and injury patterns. This is discussed further below and in more detail in other chapters.

As mentioned, sensitivity defines the amount of energy required to initiate the decompensation of the material and subsequent explosion. A primary explosive is one that is particularly sensitive to heat and/or shock and is typically used as an initiator, whereas a secondary HE is one that requires a much larger amount of energy to begin an explosion. Primary HEs are less stable, but, in spite of this, they have been used as the main charge in many bombings, most notably a type of very powerful homemade explosive (HME) called triacetone triperoxide (TATP). Secondary HEs, such as TNT, dynamite, and plastic explosives, are designed to be less sensitive to shock and heat so that they are safer for regular use.

Primary explosives, such as lead azide or diazodinitrophenol (DDNP), which are commonly used in blasting cap initiators, can be used as a primer for a larger explosive device utilizing a larger charge of secondary HE. When a small amount of energy is transmitted to the primary explosive (e.g., a blasting cap inserted into plastic explosive) it is exploded, providing the large initiating energy required for the secondary explosive to detonate. This chain reaction is known as the “explosive train.”

Some experts include the category of tertiary HEs, which require such a substantial amount of energy to detonate that even a typical initiator such as a blasting cap is insufficient. Instead, a “booster” of primary or secondary explosive is needed to detonate the entire device. A very common tertiary explosive used in mining and industry is ANFO (ammonium nitrate, fuel oil mixture). In the 1995 Oklahoma City Bombing, a similar fertilizer mixture using nitromethane and diesel fuel as the main charge was detonated using Tovex booster charges (very similar to dynamite), which were initially triggered by time-delayed nonelectric blasting caps: thus, a three-step explosive train.

Containment

Not all bombs require a container to function effectively. However, the destructive forces may be greatly increased in some cases if a special containment system is created. Containers are much more important in bombs using weaker LE as the main charge. A brick of naked C-4, a powerful HE, will detonate and create a destructive blast pressure wave that can injure people and destroy structures within the blast radius. However, LEs, such as fireworks and gunpowder, do not produce blast pressure waves when they deflagrate. If a naked pile of gunpowder is ignited, it will burn very rapidly, but the destructive force is minimal compared with that of an HE.

However, if that gunpowder is stored inside of a high-strength container, such as a lead pipe or a pressure cooker, the resulting damage is significantly greater. Using such a container can increase the destructive power of LEs in three important ways. First, and most importantly, a sturdy container allows the pressure generated by the rapidly expanding hot gases from a deflagrating LE to build up inside a fixed volume. The pressure will continue to rapidly build for as long as the container can withstand. Once a critical pressure has been reached, the structural integrity of the container will fail, and the pressure will release very quickly in an explosion that is far more powerful than it would be if the same LE were burned “naked.” There is still no blast wave generated, as the decomposition is subsonic, but the destructive force is greatly magnified.

In addition to increased explosive force, there are two other important variables that containers allow: fragments and shrapnel. Simply put, fragmentation is the physical destruction of the container in which the explosive was stored, and these container pieces themselves become ballistic projectiles called fragments . Shrapnel are pieces of material added to a bomb for the specific purpose of becoming ballistic projectiles and increasing the deadliness of the bomb. IED makers often use small metallic objects, such as nails, ball bearings, rocks, etc., packed inside the container, to cause maximum damage. This technique was used with devastating results in the Boston and Atlanta bombings.

Importantly, not all IEDs are made completely from scratch. Outside of the United States, IEDs are often created using previously mass-manufactured weapons, such as artillery shells, land mines, and other military ordnance. This ordnance typically uses trinitrotoluene (TNT) or some derivative of cyclonite (RDX) type of plastic explosive as the main charge, as originally created in the factory. The weapon is simply repurposed in an improvised manner by creating a new type of trigger system or deployment method. A classic example is the use of an unexploded HE artillery shell that is repurposed as a hidden roadside bomb, after attaching an electrical circuit to the intact explosive train, and triggered by cell phone or other remote switch. These types of IEDs were seen almost daily during the earlier years of the Global War on Terrorism (GWOT) in Iraq and Afghanistan.